High affinity, stereospecific recognition sites (receptors) for neurotransmitter, neuromodulators, and many clinically useful drugs have been identified in both peripheral tissues and the central nervous system. The interaction of a neurotransmitter, neuromodulator or drug with a specific recognition site initiates a series of events (for example, the opening of an ion channel or activation of an enzyme) resulting in either a physiological response (in the case of a neurotransmitter or neuromodulator) or a pharmacological effect (in the case of a drug). Furthermore, the presence of recognition sites for synthetic substances indicates that endogenous substances may also be present which can mimic (or antagonize) the effects of exogenously applied (synthetic) compounds. Studies are in progress to characterize "recognition-effector" systems with special emphasis on ligand (transmitter)-gated ion channels, to link novel recognition sites to their appropriate effector systems, to isolate and identify novel endogenous ligands of physiological and pathophysiological importance, to develop appropriate model systems to examine these phenomena, and to relate recognition-effector systems to both physiological and pathophysiological processes.